Bubble cycling as the explanation of the promotion of ultrasonic cavitation in a rotating tube exposure system.

The cavitation-promotion effect of rotating a sample tube during ultrasound exposure is not yet fully understood. Cavitation-induced hemolysis was observed in 0.5% hematocrit suspensions of erythrocytes exposed for 5 min at 37 degrees C. For 1.61 MHz unfocused exposure an apparent threshold of 4 W/cm2 spatial-peak intensity was obtained for hemolysis in a 72 rpm rotating exposure chamber which was made of dialysis tubing. If the tube was not rotated, then no hemolysis was observed up to 16 W/cm2. Creation of standing waves in the sample by using a thick-walled culture tube, or by placing a styrofoam reflector behind the dialysis tube did not change the apparent threshold. Focused 1.45 MHz exposure gave the same apparent threshold in a rotating dialysis tube, but this increased to about 128 W/cm2 for the nonrotating case. When the focused ultrasound was turned on for half a rotation and off for half, results were similar to those for a nonrotating CW case, while on-off cycles synchronized to a quarter rotation gave results similar to the rotating CW case. A plausible explanation of the rotation effect, given these results, is that rotation brings bubbles on the rear wall, where the beam exits the tube, back around to the front again, where the bubbles may then re-nucleate the suspension, and thereby promote cavitation bioeffects in the rotating tube.